Copyright Authorization
A portion of the disclosure of this patent document contains material which is subject to copyright protection. The copyright owner has no objection to the facsimile reproduction by anyone of the patent disclosure, as it appears in the U.S. Patent and Trademark Office patent files or records, but otherwise reserves all copyright rights whatsoever.
1. Field of Invention
The field of the present invention relates in general to tunable external cavity lasers, and particularly to an improved tuning system which suppresses mode-hopping and reduces unwanted feedback.
2. Description of the Related Art
Tunable external cavity diode lasers (ECDLs) are widely used in lightwave test-and-measurement equipment and are becoming recognized as essential components for the rapidly expanding fields of wavelength division multiplexed (WDM) optical voice and data communications. The many applications within these fields represent many different sets of performance specifications. However, the following requirements are typical: small size of the optomechanical assembly; phase-continuous tuning of the wavelength; servo control of the wavelength; controllable frequency modulation (FM) at audio rates (e.g., 100 Hz to 10 kHz) in order to broaden the linewidth; and ability to generate highly linear wavelength sweeps.
Commercial ECDLs come in a variety of tunable configurations. Generally, either a retroreflector, a diffraction element, or an interference element is utilized for controlling the feedback wavelength to the gain medium and thus the output wavelength of the laser. In a first of these tunable configurations, e.g., a "Littman" configuration, tuning is accomplished by motion of either a diffraction grating or a retroreflector positioned on an actuated pivot arm. The arm, which is rather long and massive, is required to provide phase-continuous tuning. The arm is driven by either a rotary stepper motor or a rotary DC motor. The stepper motor solution provides long-range travel and built-in position encoding, but its bandwidth is too low for FM and its discrete nature is unsuitable for linear FM sweeping. The DC motor can generate linear sweeps provided it has sufficient torque, but it cannot be used for FM. In a second configuration, the tuning is accomplished by an interference-filter which is rotated with respect to the optical path. A laser with this second configuration is described in the article by Dr. P. Zorabedian et al. entitled: "Interference-filter-tuned, alignment-stabilized, semiconductor external-cavity laser", 13 Optics Letters pp. 826-828 (1988). This laser configuration as well relies on actuators, such as stepper motors, which are not suitable for FM modulation. In both the first and second configuration, FM modulation is typically performed by modulating the injection current to the semiconductor optical gain medium, which has the highly undesirable effect of introducing amplitude modulation (AM) on the laser's output.
What is needed is a new type of external cavity laser that has wide-range continuous tuning, possesses a compact, robust, inexpensive form factor, and is FM tunable.